EDM
stands for electrical discharge machining, the applications best suited
for this metal removal process are those characterized by extremely exacting tolerances
and situations that would be extremely difficult or impossible to handle with
any other method of machining.

An Overview Of EDM

The
origin of electrical discharge machining goes back to 1770, when English
scientist Joseph Priestly discovered the erosive effect of electrical
discharges. In 1943, Soviet scientists B. Lazarenko and N. Lazarenko had the
idea of exploiting the destructive effect of an electrical discharge and
developing a controlled process for machining materials that are conductors of
electricity.

With
that idea, the EDM process was born. The Lazarenkos perfected the electrical
discharge process, which consisted of a succession of discharges made to take
place between two conductors separated from each other by a film of non-conducting
liquid, called a dielectric. The Lazarenkos achieved a form of immortality with
this circuit, which today bears their name. Today, many EDMs use an advanced
version of the Lazarenko circuit.

How It Works

During
the EDM process, a series of non-stationary, timed electrical pulses remove
material from a workpiece. The electrode and the workpiece are held by the
machine tool, which also contains the dielectric. A power supply controls the
timing and intensity of the electrical charges and the movement of the
electrode in relation to the workpiece.

At
the spot where the electric field is strongest, a discharge is initiated. Under
the effect of this field, electrons and positive free ions are accelerated to
high velocities and rapidly form an ionized channel that conducts electricity.
At this stage current can flow and the spark forms between the electrode and
workpiece, causing a great number of collisions between the particles. During
this process a bubble of gas develops and its pressure rises very steadily
until a plasma zone is formed. The plasma zone quickly reaches very high
temperatures, in the region of 8,000 to 12,000' Centigrade, due to the effect
of the ever-increasing number of collisions. This causes instantaneous local
melting of a certain amount of the material at the surface of the two
conductors. When the current is cut off, the sudden reduction in temperature
causes the bubble to implode, which projects the melted material away from the
workpiece, leaving a tiny crater. The eroded material then resolidifies in the
dielectric in the form of small spheres and is removed by the dielectric. All
this without the electrode ever touching the workpiece! Making EDM a no-contact
machining process allowing you to achieve tighter tolerances and better
finishes in a wide range of materials that are otherwise difficult or
impossible to machine with traditional processes.

Growth of EDM

EDM
has rapidly earned its place alongside milling and grinding equipment as a
proactive, mainstream technology. EDM is best known for its ability to machine
complex shapes in very hard metals. The most common use of EDM is machining
dies, tools and molds made of hardened steel, tungsten carbide, high-speed
steel and other workpiece materials that are difficult to machine by
"traditional" methods.

The
process has also solved a number of problems related to the machining of
"exotic" materials such as Hastelloy, Nitralloy, Waspaloy and
Nimonic, which are used on a large scale in the aeronautical and aerospace
industries.

With
the reduction in electrode wear and increased sophistication of EDM controls in
rams, new EDM processes use simple-shaped electrodes to 3D mill complex shapes.
EDM also is being used for polishing small, intricate surfaces.

Since
EDM does not involve workpiece/tool forces like a mill or grinder, it is
possible to EDM shapes that would break conventional cutting tools or be broken
by them.

Different
Types of EDM

RAM
EDM

RAM EDM, also known as
plunge EDM or standard EDM, is the oldest form of EDM machining. It generally
consists of an electrode usually made out of graphite that is plunged into a
workpiece in order to create a blind-shaped cavity. It can also be used to
generate through holes and geometry but these are not the primary uses of the process.

Drill
EDM

Drill EDM uses rotating
concentric electrodes to drill through a workpiece and basically performs the
same functions as a drill press, except that the material hardness is not a
factor and the accuracy of the finished hole is far superior than what any
drill press can produce. It is best used to drill start holes for the wire EDM
in already hardened material as well as accurate very small holes for
industries like aerospace and medical equipment. For reference, our electrode
diameters range from .006” to .250”.

Wire
EDM

Wire
EDM uses a traveling wire electrode (usually .010” diameter or smaller) that
goes through the workpiece. The wire, in this case, is controlled by computer following
the assigned geometry for the part to be produced. At Mercatech, Inc., with the help of our fully
programmable 4 axis EDM equipment, we can produce larger components faster and
with greater precision, as well as a higher degree of economy and flexibility
for all your EDM machining needs. Our non-traditional integrated approach, as
well as technological advantages, can provide you with the best and most
profitable solutions. By helping you make the right decisions up front...
before machining... your work will be done faster and more cost-effectively
than you ever thought possible.

When Should You EDM?

This question can be
answered either by looking at geometries produced or materials machined.
Hereafter, are two summary tables to help you determine the benefits of EDM for
your applications.

Table I - When To EDM By Geometry

When

Why

Very
thin walls

No
contact, no force, no deformation

Internal
radii equal to or less than 1/32 inch parallel to tool axis

Radius is
as small as the spark gap. Generally, tool is not rotated.

High
ratios of cavity depth to width, for example, slots and ribs

No force
means very think, long elecrodes can be used.

Non-round
cavitites/openings

Electrodes
don't have to rotate.

Intermittent
cuts

No
contact, no force

Very
small parts
(fit in a 0.25-inch cube)

Easy to
fixture since no force or vibration is involved

Recessed
cuts

Cutting
tools couldn't reach cutting area or generate desired shape.

Requires
special/unique cutting tools

Electrodes
often less costly than special cutting tools. Electrode is easy to machine,
unlike carbide. Wire is available standard.

Accuracies
that are difficult to hold, maintain after heat treating (stress relieving,
and so on)

Can EDM
conductive materials of any hardness

Different
geometry at top and bottom

Wire EDM
cuts ruled surfaces with a simpler program and machine than milling.

Complex
shapes

Easier
to program because you are using a tool of constant dimension instead of a
variety of different diameter milling cutters.

Requires
multiple component assemblies

Use
taper or recess or depth: diameter capability to make it one piece.

Lower
chip/workpiece mass ratio. Slugs from wire EDM may be reusable whereas chips
from conventional machining are recyclable at best.

Certain
explosive or flammable materials

EDM
takes place under water.

Material
with hazardous dust particles

Particles
are flushed away to the filter. Reduced risk of fumes.

Note:
Workpiece material must be electrically conductive or semi-conductive with no
non-conductive cutting zones.

Tables by Charmilles Technologies

EDM for tooling applications

When
a part requires special/unique conventional cutting tools. Electrodes are easy
to machine, unlike carbide. Equally important, the wire used by a wire EDM is
available as a standard, off-the-shelf component. EDM is a low cost tooling
option when you need short run stamping (under 5,000 pieces) and low volume
broaching. With EDM, there's no need to make a die set. That's why EDM is used
to make sewing machine components and prototypes. Instead of using expensive
broaches, EDM is a very attractive form of low-cost tooling. This is a reason
companies use EDM to produce splines and gear teeth along with all their metal
stamping & mold making needs.

Limitations Of EDM

Clearly,
the benefits of EDM are considerable, and it is often appropriate to EDM
instead of using conventional manufacturing processes. But not always.
What are some of the restrictions of EDM?

Wire EDM tapering: The maximum
taper angle is ±45 degrees.The maximum height/angle is 30 degrees at 16
inches high. in The maximum electrical resistance for workpiece and
fixture is approximately 0.5-5.0 ohm centimeter for both wire and sinker
EDMs.

The accuracy of an EDM is
limited to about ±0.0001 inch for wire and ram EDMs.

Surface finish is about VDI of
0 (4 microinch) for wire and VDI of -5(2 microinch) for sinkers.

Surface integrity is 20
millionths of an inch recast layer thickness for wire and ram EDMs and 20
millionths microcrack length for wire and ram EDMs. The result can be as
good or better than a ground surface.

Sample
Applications

Tooling: Speaker Die for Car Stereo
System

Machining: Side Burn in .025” wall
thickness

Finally….

If in
doubt about any aspect of the EDM process or workpiece preparation, give us a
call!

Our many
years of experience in the field will be used to answer even your toughest
questions.